Extrusion aids for extrudable solid compositions



United States Patent EXTRUSION AIDS FOR EXTRUDABLE SOLID COMPOSITIONS Charles C. Bice, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing Filed Dec. 19, 1958, Ser. No. 783,646

15 Claims. (Cl. 52-.5)

This invention relates to the extrusion of solid propellant compositions. In one aspect, this invention relates to improved extrudable solid propellant compositions. In another aspect, this invention relates to the use of complex phosphate compounds to improve the processability and extrudability of solid propellant compositions.

Recently, it has been'discovered thatsuperior solid propellant compositions are obtained comprising a solid oxidizer comprising a solid inorganic oxidizing salt such as ammonium nitrate or ammonium perchlorate, combined with a rubbery binder material such as a copolymer of butadiene and a vinylpyridine or other substituted heterocyclic nitrogen-base compound, which after incorporation is cured by a quaternization reaction or a vulcanization reaction. Solid propellant compositions of this nature and a process for their production are disclosed and' claimed in copending application Serial No. 284,447, filed April 25, 1952, by W. B. Reynolds and J. E. Pritchard.

When propellant compositions, such as those described above, are prepared, high concentrations of solid oxidizer amounting, for example, to 84 to 94 parts with correspondingly 15 to parts of binder and 1 part of burning rate catalyst per 100 parts propellant are frequently desired to obtain a composition having a high specific impulse and for other reasons. However, these compositions or similar compositions with supplemental ingredients such as high energy, solid fuels, etc., can be diflicult to form into propellant grains by conventional extrusion methods.

In fact, some compositions can not be formed by ordinary extrusion methods. Further, if attempts are made to improve workability by incorporation of the required amounts of liquid plasticizers, the plasticizer can bleed from the composition resulting in a propellant grain having a tacky surface.

In my copending application Serial No. 743,456, filed June 20, 1958, I have disclosed and claimed propellant compositions containing at least 86 parts by weight of ammonium nitrate together with finely divided, high-' energy additives, such as boron or magnesium. The flow properties of such propellant compositions having the described high solid oxidizer contents are such that the compositions cannot be extruded 'by conventional methods. r

I have now found that such solid oxidizer content propellant compositions can be extruded in conventional extrusion apparatus by incorporating therein an extrusion aid comprising certain complex ortho phosphate compounds. The extrusion aids which are employed are orthophosphate compounds described as double salts and as solid solutions represented by the formula In this formula, M is a metal selected from the group calcium, zinc and strontium and both M atoms are alike; and x has a value between 0.5 and 3.0. The

Patented Dec. 20, 1960 The above compounds are sometimes described in terms of the mol ratios of the metal oxide and phosphorous pentoxide, exclusive of the hydroxide. For example, the second compound in the above list, which is sometimes called basic calcium phosphate, can be described as 3CaO-P O 'Ca(OH) The compound 3[Ca (PO ]-Ca(OH) is named hydroxylapatite (sometimes spelled hydroxyapatite) and can be described as 9CaO-3P O -Ca(OH) It is an object of this invention to provide extrudable solid propellant compositions having a high ratio of solid inorganic oxidizer to binder material. It is also an object of this invention to provide a method for rendering high solids content propellant compositions amenable to conventional extrusion methods. It is a further object of this invention to provide an improved extrusion aid for use with solid propellant composi-' tions. Other objects and advantages will be apparent to one skilled in the art upon reading this disclosure including the detailed description of the invention.

Hereafter, in the specification and claims the term extrudable compositions is meant to cover only those compositions which, when formed by extrusion into the desired shape, maintain the extruded dimensions within a tolerance of plus or minus 5 percent and preferably within plus or minus 2 percent even prior to curing by thermal or other means. For propellant compositions which require thermal cure, these tolerances are maintained at temperatures as high as generally encountered in thermal cure, including to 250 F. ,Although the invention is applicable to both extrudable and moldable compositions, it displays particular utility in compositions intended for extrusion processes.

The complex phosphate compounds of this invention can be used to improve the processability and extrudability of solid propellant compositions without posing any serious problem to the propellant manufacturer in that no changes need be made in the processing details. The compounds which comprise the extrusion aids of the invention are solid, free-flowing powders and they are, therefore, convenient to use. These compounds preferably are added simultaneously with or in admixture with the oxidizer. The amount of complex phosphate extrusion aid which can be used is commonly in the range of 0.1 to 10 percent by weight of the propellant composition and preferably is used in the range of 0.2 to about 5 percent by weight of propellant composition because this range is particularly effective and does not materially alter the propellant composition. Be cause of the effectiveness of these compounds, it is possible to prepare with greater ease than heretofore possi,

apes-res ble, propellants having a high percentage of solid inorganic oxidlzer, for example, concentrations as high as about 94 percent by weight of propellant. The complex phosphate extrusion aids of this invention should be finely ground so as to have a particle size of less than 200 microns and preferably an average particle size of less than about 50 microns so as to aid in obtaining uniform distribution of the extrusion aid.

The rubbery materials which are applicable for use as the binder and fuel components of the compositions of this invention include natural and synthetic rubbery polymers which commonly are characterized by Mooney values (ML-4) in the range of from about to about 75. Types of rubbery materials which are useful include: natural rubber, silicone rubber, polybutadiene, butadienestyrene copolymers, butadiene-acrylonitrile copolymers, and copolymers of butadiene with a copolymerizable heterocyolic nitrogen base of the pyridine and quinoline series containing a CH =C group such as 2-methyl-5- vinylpyridine. Commonly preferred rubbery binders are copolymers prepared by copolymerization of the major amount of a conjugated diene, preferably 1,3-butadiene, with a copolymerizable monomer containing the CH =C group such as Z-methyl-5-vinylpyridine. The 90/10 copolymer of l,3-butadiene/2-methyl-5-vinylpyridine has been found to be particularly suitable and is frequently preferred.

These rubber polymers are compounded, as is wellknown in the rubber art, with a reinforcing filler such as carbon black, with antioxidants, with plasticizers, and with curing agents such as vulcanizing or quaternizing agents.

Commonly amounts up to 100 parts by weight of carbon black per 100 parts by weight of rubber are used as a reinforcing filler and fuel.

The plasticizers include the known rubber plasticizers such as compounds including dioctyl phthalate, benzophenone, amylbiphenyl, di(butoxyethoxyethoxy) methane, trioctyl phosphate, tricresyl phosphate, and liquid polymers of 1,3-butadiene. These plasticizers are commonly used in amounts of 0 to parts by weight per 100 parts of rubber. The plasticizer is generally used in amounts sufficient to soften the rubber so as to facilitate preparation of the binder. In one embodiment of this invention, part of the plasticizer is added with the phosphate compounds herein disclosed and mixed with the oxidizer.

Curing agents for use with rubbery copolymers are Well-known in the art and include sulfur, sulfur'liberating materials such as thiuram disulfides, polysulfides, alkylphenol sulfides and N,N'-dithioamines. Non-sulfur curatives are also known. Accelerator-activators are .frequently used in conjunction with the curing agents and examples of such compounds include lead oxide, zinc oxide, magnesium oxide, mixtures of magnesium oxide and carbon, lead carbonate, hydrated lime, lead silicate, dibutyl ammonium oleate, oleic acid, dibenzylamine, linseed oil, fatty acids, rosin acids, triethanolamine, zinc stearate, and the like. When the copolymers are prepared from a mixture of monomers which includes a heterocyclic nitrogen base, as hereinbefore described. quaternizing agents are sometimes used as the curing agent. These quaternizing agents include various alkyl halides such as methyl iodide, ethyl iodide, hexyl iodide, oc tyl bromide; various alkylene halides such' as propylenetchlo ride; various substituted alkanes which contain at least one hydrogen attached to the carbon atom attached to a halogen such as chloroform, 1,3-dichloropropane, ethyl ene chlorohydrin; various substituted aromatic compounds such as picryl chloride, benzyl chloride, benzene sulfonyl chloride, benzal chloride, benzotrichloride, methylbenzene sulfonate; and various polyhalogenated cycloalkanes such as hexachlorocyclopentadiene, and hexachloro-para-xylene. i

Antioxidants commonly used in rubber compounding can be employed and these include hydroquinone, monobenzyl ether, phenyl-beta-naphthylamine, polymerized trimethyldihydroquinoline, heptylated diphenylamine, monoesters of salicyclic acid, hexachloronaphthalene, hydrocarbon waxes, etc.

The above list of rubbery polymers, plasticizers, and other compounding ingredients is illustrative but is not exhaustive of the compounds which can be employed and is not to be considered as limiting. The rubbery polymer, together with such compounding ingredients as are required or desired to obtain a specific result comprise that which is designated as the binder component of the propellant composition. In the practice of the present invention, the binder is commonly used in amounts of 5 to 15 parts by weight for correspondingly 84 to 94 parts by weight of one or more solid oxidizers and about 1 part of burning rate catalyst per parts of propellant.

A general formulation for a binder composition is given in Table I.

Suitable oxidizers include solid inorganic oxidizing salts such as one or more of the ammonium and alkali metal salts of nitric and perchloric acids including ammonium nitrate, potassium nitrate, sodium nitrate, lithium nitrate, cesium nitrate, rubidium nitrate, ammonium perchlorate, potassium perchlorate, and the like. Phase stabilized ammonium nitrate which comprises about 6 to 15 percent by weight of potassium nitrate in admixture with the ammonium nitrate is frequently a preferred oxidizer because such stabilized ammonium nitrate is less sensitive to temperature changes. These inorganic oxidizing salts can be used with minor amounts of organic oxidizers and modifiers.

Burning rate catalysts include iron oxide, ammonium dichromate, Milori blue, and other complex cyanides including the complex cyanides of iron and nickel such as ferric ferrocyanide and ferrous ferricyanide and are used in amounts up to about 5 percent by weight of the solid propellant composition. The burning rate catalyst can be omitted if desired.

Solid, high-energy fuels, such as powedered metal including magnesium, boron, aluminum, and iron can also be used in amounts up to about 25 percent by Weight of the solid propellant composition.

The propellants which are prepared according to this invention are those illustrated in Table II.

TABLE H Composition Parts per 100 parts propellant Binder 7 Oxidizer; 63-32 Extrusion aid 0. 1-10 Catalyst- 0-5 High energy additives 0-25 parts of- Z-methyl-S-vinylpyridine. The rubbery polymer was used to prepare a binder having a composition according to the following Table III:

TABLE III Composition Parts by Weight 90/10 Co 01 er 100 Furnace crgli on Bl k 22 Antioxidant 1 3 A physical mixture containing 65 percent of a complex diarylami'w ketone reaction product and 35 percent of N ,N '-d.iphenyl-para-phenylaminedlamine.

The oxidizer used in the preparation of the propellants with the above binder was phase stabilized ammonium nitrate, produced by mixing 90 parts ammonium nitrate with parts of potassium nitrate and 0.2 part of water. The mixture was heated 24 hours at 150 F. and ground to a weight average particle size of 60 microns. The stabilized oxidizer was mixed with the burning rate catalyst (ammonium dichromate) which had a weight particle size of 18 microns. Using the above mixture of oxidizer and catalyst, the propellant compositions shown in Table IV were prepared by adding the remaining ingredients to the oxidizer and adding the resulting mixture slowly to the binder. The mixing was effected in a Baker- Perkins mixer over a period of to 30 minutes. The resulting mixture was a crumbly solid. The solid material was placed in an extruder equipped with a die having a diameter of 0.5 inch; and a pressure of 9600 p.s.i.g was applied to the propellant composition, and the rate of extrusion in inches per minute was measured.

TABLE IV Composition and extrusion rates of propellants Parts by Weight Test Composition A B C D E Components:

Bin

Catalyst Liquid Polybutadieue 1500 Liquid Polybutadiene 2500 Ca;;(PO4)2.Ca(OH) Extrusion rate, inches/second cooler-3m imam-30.

lbctoow cn UNOi-UIOI N egligible.

From soldium-catalyzed polymerization of 1,3-butadieue prepared substantially as described by Crouch et al., Ind. Eng. Chemistry, vol. 47, 2091-5 (1955). The products are designated by numbers indicating the approximate viscosity in centipoises at 100 F.

2 Monsanto reagent grade having a particle size substantially less than 44 microns. 1

Composition A of Table IV had a negligible extrusion rate and was not considered processable under the imposed conditions. Composition B which serves as the control test for composition C; and composition D which serves as the control test for composition E, shows that prior art, liquid plasticizers for rubber are not satisfactory. While higher concentrations of plasticizers might improve extrusion rates, the use of concentrations higher than about 2 parts by weight in these compositions is not desirable because the extruded products have a tacky sur- 6 face and the liquid plasticizer bleeds from the composition.

The effectiveness of the complex phosphate compounds herein disclosed is shown by the results with compositions C and B. These compositions extruded rapidly and the resulting grains were smooth and free of visual defects. Thus, the plasticizer serves to soften the material to facilitate the preparation and mixing of the composition and the complex phosphate compounds act in an entirely difierent roll, that being specifically as an aid to extrusion. The grains retain their shapes and dimensions during the curing operation at a temperature of about 150 to 250 F. for about 24 hours.

Reasonable variations and modifications are possible within the scope of the present disclosure without departing from the spirit and scope of the invention.

That which is claimed is:

1. In the process of preparing a solid propellant composition comprising a solid inorganic oxidizing salt and a rubbery binder material selected from the group consisting of natural and synthetic rubber wherein the oxidizing salt is incorporated into the binder and wherein the oxidizing salt content is about 84 to about 94 parts by weight per parts of the propellant composition, the improvement which comprises admixing with said propellant composition, during incorporation of the oxidizing salt into the binder, about 0.1 to about. 10 weight percent of the propellant composition of a complex metal phosphate compound represented by the formula wherein M is a metal selected from the group consisting of calcium, zinc and strontium and both M atoms are alike, and x has a value between 0.5 and 3.0 whereby the propellant composition is rendered extrudable.

2. The process of claim 1 wherein the complex metal phosphate is calcium hydroxyapatite according to the formula Ca (PO -Ca(OH) 3. The process of claim 2 wherein the inorganic oxidizing salt is ammonium nitrate, and the rubbery binder is a copolymer of butadiene and Z-methyl-S-vinylpyridine.

4. The process of claim 1 wherein the complex metal phosphate is represented by the formula 5. The process of claim 1 wherein the complex metal phosphate is represented by the formula 6. The process of claim 1 wherein M is calcium.

7. The process of claim 1 wherein M is zinc.

8. The process of claim 1 wherein M is strontium.

9. In the process for preparing a solid propellant composition comprising combining about 84 to 94 parts by weight of a solid inorganicroxidizing salt, about 5 to about 15 parts by weight of a robbery binder selected from the group consisting of natural and synthetic rubber, and about 0.] to about 10 parts by weight of a burning rate catalyst selected from the group consisting of iron oxide, ammonium dichromate, and complex cyanides of iron and nickel, the improvement comprising admixing with said oxidizing salt, prior to combining said oxidizing salt with the remaining components of said propellant composition about 0.1 to about 10 parts by weight per 100 parts of propellant composition of a complex metal phosphate represented by the formula wherein M is a metal selected from the group consisting of calcium, zinc and strontium and both M atoms are alike, and x has a value between 0.5 and 3.0.

10. An improved solid propellant composition comprising about 84 to 94 parts by weight per 100 parts of propellant composition of a solid inorganic oxidizing salt; about 5 to about 15 parts by weight per 100 parts of'propellant composition of a rubbery binder selected from the memes 0.1 to about 10 parts by weight per 100 parts of propellaut composition of a burning rate catalyst; and about 0.1 to about 10 parts by weight per 100 parts of propellant composition of a complex metal phosphate compound represented by the formula 14. The composition of claim 10 wherein the complex metal phosphate is represented by the formula 15. An improved propellant composition comprising about 84 to aboutt94 parts by weight per .100 parts of propellant composition of an oxidizer comprising about weight percent ammonium nitrate and about 10 weight percent potassium nitrate; about 5 to 15 parts by weight per parts of propellant composition of a rubbery binder comprising a copolymer of butadiene and 2- methyl-S-vinylpyridine; about 0.1 to about 10 parts by weight per 100 parts of propellant composition of ammonium dichromate; and about 0.1 to about 10 parts by weight per 100 parts of propellant composition of a complex metal phosphate compound represented by the formula x[M (PO ]-M(OH) wherein M is a metal selected from the group consisting of calcium, zinc and strontium and both M atoms are alike, and x has a value between 0.5 and 3.0.

OTHER REFERENCES Chem. and Eng. News, October 7, 1957, pages 62-3. 

15. AN IMPROVED PROPELLANT COMPOSITION COMPRISING ABOUT 84 TO ABOUT 94 PARTS BY WEIGHT PER 100 PARTS OF PROPELLANT COMPOSITION OF AN OXIDIZER COMPRISING ABOUT 90 WEIGHT PERCENT AMMONIUM NITRATE AND ABOUT 10 WEIGHT PERCENT POTASSIUM NITRATE, ABOUT 5 TO 15 PARTS BY WEIGHT PER 100 PARTS OF PROPELLANT COMPOSITION OF A RUBBERY BINDER COMPRISING A COPOLYMER OF BUTADIENE AND 2METHYL-5-VINYLPYRIDINE, ABOUT 0.1 TO ABOUT 10 PARTS BY WEIGHT PER 100 PARTS OF PROPELLANT COMPOSITION OF AMMONIUM DICROMATE, AND ABOUT 0.1 TO ABOUT 10 PARTS BY WEIGHT PER 100 PARTS OF PROPELLANT COMPOSITION OF A COMPLEX METAL PHOSPHATE COMPOUND REPRESENTED BY THE FORMULA X(M3(PO4)2) M(OHH)2 WHEREIN M IS A METAL SELECTED FROM THE GROUP CONSISTING OF CALCIUM, ZINC AND STRONTIUM AND BOTH M ATOMS ARE ALIKE, AND X HAS A VALUE BETWEEN 0.5 AND 3.0. 